A high-frequency package used in an electronic device that operates in a microwave band or a millimeter-wave band is fabricated by packaging a multilayer substrate having mounted thereon a high-frequency device (MMIC: Monolithic Microwave Integrated Circuit) that operates in the microwave band or the millimeter-wave band. An external electric connection of the high-frequency package is attained through a lowermost layer of the multilayer substrate.
For this purpose, a quasi-coaxial line that connects between an uppermost layer and the lowermost layer is formed in the multilayer substrate of the high-frequency package. The quasi-coaxial line is a structure for externally outputting a high-frequency signal generated by the mounted high-frequency device and for transferring an externally input high-frequency signal to the high-frequency device.
As disclosed in Patent Literature 1 for example, the quasi-coaxial line has a central conductor and outer conductors. The central conductor is a vertical through hole via that connects between a metal pattern formed on an upper surface of the uppermost layer and connected to an input/output port of the high-frequency device and a metal pattern formed on a lower surface of the lowermost layer. The outer conductors are a plurality of interlayer vias that are circularly arranged around the central conductor and connect between two or more layers. The metal pattern formed on the lower surface of the lowermost layer is connected to another multilayer substrate through a BGA (Ball Grid Array).